Prepare Nicoise Dressing. To serve, line 4 salad plates with lettuce leaves. Arrange chilled vegetables, tuna, tomatoes, eggs, and olives (if desired) on the lettuce-lined plates. Sprinkle each serving with green onions. If desired, top each salad with an anchovy fillet and garnish with tarragon. Shake dressing; drizzle over each salad. Makes 4 main-dish servings.